1. Field of the Invention
The present invention relates to a cooling device for an internal combustion engine with a fuel injector for injecting fuel directly into the cylinder, in which a current combustion is changed between pre-mixture combustion and stratified charge combustion in accordance with a current engine operating condition.
2. Description of the Related Art
In a known internal combustion engine with a fuel injector for injecting gasoline fuel directly into the cylinder, fuel injection at a low engine load is carried out during a compression stroke so that a combustible gas mixture is formed only close to the spark plug and thus a stratified charge combustion is carried out. On the other hand, fuel injection at a high engine load is carried out during an intake stroke so that injected fuel is sufficiently mixed with intake air and thereafter pre-mixture combustion is carried out. Japanese Unexamined Patent Publication No. 5-52145 discloses an internal combustion engine in which fuel injection timing is changed, as above-mentioned, and whereby, at a low engine load, lean burn can be realized so that fuel consumption can be improved. On the other hand, at a high engine load, a required high engine output can be obtained. The engine in which the stratified charge combustion and the pre-mixture combustion are alternated is usually cooled by cooling water. During the stratified charge combustion and during the pre-mixture combustion, the same cooling system is used, which has a passage in which cooling water circulates and a radiator in which heat of cooling water releases.
The cooling system is adapted to the pre-mixture combustion. Therefore, an excess cooling occurs during the stratified charge combustion. In the stratified charge combustion, a combustible gas mixture is formed only close to the ignition plug and thus combustion is carried out. Accordingly, the time from fuel injection to ignition is short. On the other hand, the temperature of the engine block is made low by the cooling so that injected fuel can not vaporize sufficiently in the short time. Therefore, a discharged amount of unburned fuel (HC) can increase. Moreover, if injected fuel can stick to the ignition plug, the fuel can not vaporize due to the low temperature of the engine block so that a misfire can occur. Furthermore, a difference in temperature between the center portion and the periphery portion of the piston becomes large so that a deformation of the piston is easily caused and thus scuffing of the piston can occur.